A communication method making use of Orthogonal Frequency Division Multiplexing (OFDM), as represented by Long Term Evolution (LTE), now being discussed in 3rd Generation Partnership Project (3GPP), attracts attention as a communication method for the next generation. OFDM is a transmission method in which a bandwidth to be used is divided into multiple subcarriers, and then each data symbol is assigned to each subcarrier. Since the subcarriers are so arranged as to be orthogonal one another on a frequency axis, OFDM is excellent at frequency usage efficiency. Moreover, each subcarrier has a narrow bandwidth, and therefore an effect of multiple-path interference can be suppressed so that a high-speed large-capacity communication can be realized.
In a data transmission method using OFDM, generally at a transmission side, already-known pilot symbols besides data symbols are inserted at intervals in the frequency-wise direction and the time-wise direction, as shown in FIG. 13. At a receiver side, amplitude-phase variation of the pilot symbols is detected from a received signal, and then data symbols included in the received signal are demodulated, on the basis of the amplitude-phase variation. The amplitude-phase variation detected from the pilot symbols is called a channel estimation value.
To calculate a channel estimation value, a Cyclic Prefix (CP) part is removed from a received signal, and Fast Fourier Transform (FFT) in accordance with an effective symbol length of OFDM is carried out in order to extract pilot symbols, and then the pilot symbols extracted are verified with an already-known pattern. In this way, amplitude-phase variation of the symbols is calculated, and the amplitude-phase variation is deemed to be a channel estimation value.
A result of transforming channel estimation values of the frequency domain into the time domain by means of Inverse Fast Fourier Transform (IFFT) is called a delay profile. In a delay profile, peaks of power appear in response to propagation paths of a transmission line. A low-level part in the delay profile means that there is no propagation path, or a propagation path there is weak, so that noise power is dominant there. After replacing values of the delay profile at its low-level part with “0”, transforming the data back into the frequency domain by means of FFT makes it possible to cancel noise components from the original channel estimation values. Moreover, a propagation delay time can be calculated according to the positions of power peaks in the delay profile.
The calculated propagation delay time is used for transmission time control. For example, if a mobile station for transmitting/receiving information to/from a base station by way of a wireless radio communication executes a transmitting process in accordance with timing of a received signal from the base station as it is, the transmission signal is received twice the one-way propagation time late, at the base station. In the meantime, the base station transmits/receives information to/from a plurality of mobile stations; and therefore, if a distance between the base station and each mobile station is different to others, there appears a time-wise overlap in received signals from the mobile stations so that interference occurs.
Explained below with reference to FIG. 14 and FIG. 15 is interference that occurs in the case where a mobile station executes a transmitting process in accordance with timing of a received signal from the base station as it is. For an easy understanding, an explanation is made here with a case example of transmission/receiving between one base station BS and two mobile stations MS1 and MS2.
As shown in FIG. 14, it is assumed that the mobile station MS1 receives a receiving frame “A” from the base station BS, and transmits a transmission frame “a” to the base station BS. Meanwhile, it is assumed that the mobile station MS2 receives a receiving frame “B” from the base station BS, and transmits a transmission frame “b” to the base station BS. Under the situation, if the mobile stations MS1 and MS2 transmit the transmission frame “a” and the transmission frame “b” in accordance with timing of signal receiving from the base station BS as it is, each transmission signal from the mobile stations MS1 and MS2 is received twice the one-way propagation time late, at the base station BS, as shown in FIG. 15.
Herein, a propagation delay time of the receiving frame “A” at the mobile station MS1 is defined as “t1”, and a propagation delay time of the receiving frame “B” at the mobile station MS2 is defined as “t2”. The transmission frame “a” from the mobile station MS1 is received “2t1” late at the base station BS, wherein “2t1” is twice a time needed for a propagation from the base station BS to the mobile station MS1. On the other hand, the transmission frame “b” from the mobile station MS2 is received “2t2” late at the base station BS, wherein “2t2” is twice a time needed for a propagation from the base station BS to the mobile station MS2. In this case, if the distance between the base station BS and the mobile station MS1 and the distance between the base station BS and the mobile station MS2 are different from each other, needed times for respective propagations are different (t1≠t2). Accordingly, at the base station BS, there appears a time-wise overlap in the transmission frame “a” from the mobile station MS1 and the transmission frame “b” from the mobile station MS2, so that interference occurs.
Explained next with reference to FIG. 16 and FIG. 17 is an adjustment of transmission timing at each of the mobile stations MS1 and MS2.
A propagation delay time measured at the mobile station MS1 is defined as “t1”, and a propagation delay time measured at the mobile station MS2 is defined as “t2”. Under this situation, the mobile station MS1 puts a transmission start timing of the transmission frame “a” in relation to the receiving frame “A” ahead twice the measured propagation delay time “t1”, as shown in FIG. 16. In the meantime, the mobile station MS2 puts a transmission start timing of the transmission frame “b” in relation to the receiving frame “B” ahead twice the measured propagation delay time “t2”, as shown in FIG. 16. According to this arrangement, each transmission frame coming from the mobile station MS1 and the mobile station MS2 is received without causing a time-wise overlap at the base station BS, accordingly having no interference, as shown in FIG. 17.
A measurement of a propagation delay time is explained below with reference to a flowchart of FIG. 18. In the following explanation, a mobile station, a transmit/receive terminal to be used as a mobile station, and a device to be used as a receiving unit of those mobile station and terminal are collectively called a “receiver device.”
In order for a measurement of a propagation delay time, channel estimation values are calculated at first. For calculating a channel estimation values; CP is removed from a received signal (Step S21) as described above, FFT tailored to an effective symbol length of OFDM is carried out (Step S22), and pilot symbols are extracted (Step S23). Then, the extracted pilot symbols are verified with an already-known pattern, and an amplitude-phase variation of the symbols is calculated so as to obtain the channel estimation values (Step S24).
To measure a propagation delay time in accordance with the channel estimation values, the channel estimation values in the frequency domain is transformed into the time domain by means of IFFT (Step S26), and positions of power peaks are detected from a delay profile obtained (Step S27). Then, the propagation delay time can be calculated according to the positions of power peaks.
Moreover, with respect to a low-level part in the delay profile, by way of replacing values of the delay profile with “0” (Step S28) and transforming back into the frequency domain by using FFT (Step S29), it becomes possible to cancel noise components from the original channel estimation values.
At the time of carrying out IFFT with respect to channel estimation values in the frequency domain; the number of channel estimation values, namely pilot symbols, is sometimes not a power of 2. In such a case, IFFT of the number of samples of the channel estimation values is not carried out but IFFT of size larger than the number of samples of the channel estimation values is carried out. At the time, the channel estimation values are so placed as to fill a front end and its subsequent followers; and for any part with an insufficient number of samples is not filled with 0 but the channel estimation values are interpolated (Step S25). Thus, a signal power of a part with no propagation path decreases in the delay profile so that a canceling process on noise components can be improved (for example, refer to NPL 1).
Moreover, a technology of using an arrangement pattern of already-known samples, in which a noise effect is canceled by means of carrying out IFFT samples of the already-known pattern included in a received signal, is also known as a technology for calculating a delay profile (for example, refer to PTL 1).